The present invention relates to a process for the preparation of highly aromatic reformates which are suitable for use in blending high aromatic content motor gasolines and/or for the production of high purity aromatic petrochemical products by high-severity reforming. More particularly, the present invention relates to a process for the conversion of naphtha stocks by high-severity catalytic reforming, under conditions in which the balance between the various reforming reactions is controlled by catalyst modification, to produce a reformate of heretofore unobtainable aromatic content and yields, and which is of sufficient quality to produce commercially acceptable purity aromatic hydrocarbons directly upon separation by fractional distillation.
The process of the present invention is particularly efficacious for the production of C.sub.6 to C.sub.8 aromatic hydrocarbons with outstanding purity and yield. As will be understood by those skilled in the art, the term "C.sub.6 to C.sub.8 aromatic hydrocarbons" as used herein refers to aromatic hydrocarbons having 6 to 8 carbon atoms per molecule, and includes such aromatic hydrocarbons as benzene, toluene, and xylene. As also used herein, the term "xylenes" refers to the C.sub.8 aromatic hydrocarbons in a generic sense and includes para-xylenes, metal-xylenes, orthoxylenes and ethylbenzene. Moreover, as further used herein, the term "highly aromatic reformates" refers to those reformates which are of sufficient quality to yield C.sub.7 and/or C.sub.8 aromatic hydrocarbons of commercially acceptable quality directly upon fractional distillation without the necessity for solvent extraction or extractive distillation. Generally, such reformates will have a research clear octane value of at least about 100. Accordingly, the present invention contemplates the production of reformates of at least about 100 research clear octane. Reformates of this octane value are highly useful in the production of high purity aromatic hydrocarbons, and are also highly advantageous for use as blending stocks in the preparation of high octane, lead-free motor gasolines having a high aromatic content.
In the production of aromatic hydrocarbons, it is well known that naphthas contain large amounts of naphthenes which can be catalytically reformed to aromatic hydrocarbons, and particularly C.sub.6 to C.sub.8 aromatic hydrocarbons under conditions effective for dehydrogenation, isomerization, and dehydrocyclicization. Heretofore, however, the presence of nonaromatic, and particularly paraffinic materials, in the feed stock which boil in the same range as the desired aromatics has posed signifcant obstacles to the production of high-purity aromatic hydrocarbons in high yields. In conventional reforming processes, significant quantities of thse nonaromatic materials are not substantially converted to aromatics and/or cracked to lower-boiling, easily removable compounds. Consequently, reformates produced under conventional reforming conditions contain significant amounts of paraffins which cannot be separated from the aromatic hydrocarbons by low-cost separation techniques, such as fractional distillation, but only with great cost and difficulty such as by solvent extraction or extractive distillation. Accordingly, in order to produce a C.sub.6 to C.sub.8 aromatic hydrocarbon product of commercial quality, it is conventional to subject the resultant reformate to a costly solvent extraction step. Due to the large cost attendant solvent extraction, and the additional manpower required therefor, the prior art has sought to develop processes for the production of reformates which do not require a solvent extraction step in order to produce an aromatic hydrocarbon product of commercially acceptable quality.
Generally, these prior art processes have involved reforming the naphtha stocks under reforming conditions of high severity in order to crack the paraffins to easily removable gaseous hydrocarbons. In conventional high-severity reforming processes, however, the high severities necessary to produce a reformate having a concentration of paraffins sufficiently low to yield high-purity aromatic hydrocarbons without solvent extraction has also resulted in the cracking of significant quantities of naphthenes, with a concomitant decrease in yield in the aromatic product. Hitherto, therefore, conventional high severity reforming processes have been unable to realize the C.sub.6 to C.sub.8 aromatic hydrocarbons in significant yields.
One approach to this problem has been to prefractionate the naphtha feed stock into very narrow boiling range heartcuts containing only those aromatic precursors which have a lower boiling pont than the aromatics to be produced therefrom in order to allow the facile separation of the reformate into unconverted nonaromatic material and a mixture of C.sub.6 to C.sub.8 aromatic hydrocarbons. By employing such prefractionations, the amount of difficulty crackable paraffinic material is reduced, and consequently the reforming process may be operated under less severe reforming conditions, thereby reducing the volume loss resulting from high-severity reforming. For example, in U.S. Pat. No. 3,635,815, a naphtha feed fraction is prefractionated into an overhead fraction having an upper endpoint of 270.degree. to 275.degree. F. and a bottoms fraction having a higher endpoint. The overhead fraction is then catalytically reformed under reforming conditions of severity sufficient to convert any remaining paraffins to easily removable compounds. The resulting reformate is then subjected to a plurality of fractionation steps to produce a mixture of high-purity C.sub.8 aromatic hydrocarbons.
While processes employing prefractionation steps produce reformates which yield aromatic hydrocarbons of adequate purity, upon fractionation these processes still achieve less than desirable yields. Prefractionation of the naphtha feed stocks into such very narrow boiling range fractions removes significant quantities of aromatic hydrocarbon precursors from the conversion process and correspondingly reduces the yield of C.sub.6 to C.sub.8 aromatic hydrocarbons per volume of naphtha feed.
It is also known in the art that the amount of naphthene destruction may be reduced somewhat by employing a two-step reforming process in which a naphtha feed is reformed under mild conditions in a first step and then thermally cracked in a second step. Even with the use of a two-step reforming process, conventional reforming processes produce a reformate having an aromatic hydrocarbon concentration and yield which is less than desirable. Since even under mild conditions conventionally employed reforming catalysts promote a significant amount of naphthene cracking, the yield of aromatic hydrocarbons per volume of naphthene feed is still less than desirable. Moreover, even under high-severity reforming conditions, conventionally employed reforming catalysts do not promote the cracking of nonaromatic, and particularly paraffinic, material with substantial completion. Accordingly, an aromatic product of less than desirable yield and purity is obtained from conventional processes, even with the utilization of a two-step reforming system. For example, in U.S. Pat. No. 3,499,945, the combination of a prefractionation step and a two-step reforming process is necessary to achieve a reformate of sufficient quality to produce a toluene product of commercially acceptable purity.
Cox. U.S. Pat. No. 2,642,384 teaches that the hydrocracking activity of a conventional platinum-halogen-alumina reforming catalyst may be increased by increasing the halogen content thereof. In patentee's invention, this discovery is employed to maintain the hydrocracking activity of the reforming catalyst constant throughout a reforming system by adding a small amount of a halogen compound to a reforming zone and reacting it with the reforming catalyst contained therein. Cox further teaches that this discovery may be employed to alter the quality of the reformate being produced. However, in Cox, catalyst modification is employed only to uniformly modify the entire catalyst inventory of the reforming system. Moreover, Cox is directed to the upgrading of gasolines, and not to the production of reformates suitable for use in the preparation of high-purity aromatics. In the upgrading of gasolines, it is desirable to convert the heavy paraffinic material to high octane branched paraffins. The formation of reformates containing a high percentage of desirable isoparaffins is incompatible with the production of reformates suitable for the preparation of high-purity compounds since the branched paraffins have a similar boiling range with the C.sub.6 to C.sub.8 aromatics, and consequently would preclude the production of the aromatic compounds in pure form without extensive subsequent purification.